The present invention relates generally to centrifugal contactors for separating liquids of different weight phases, and more particularly to such centrifugal contactors provided with vane sets in a mixing zone for the process liquids for enhancing the mixing of the liquids and for urging the liquid in the mixing zone toward the process liquid inlets.
Centrifugal contactors have been found to provide a highly satisfactory mechanism for separating liquids from one another based on different weight phases. The use of centrifugal contactors is particularly advantageous in operations where short residence times, small inventories and high separating force fields are beneficial. Typical employment of centrifugal contactors where success has been demonstrated is in areas including pharmaceuticals, processing of lubricating oils, treatment of liquid wastes and nuclear reactor fuel reprocessing. Centrifugal contactors are particularly desirable for the reprocessing of nuclear reactor fuels since in such instances short residences times, small inventories and high separating force fields are of high priority due to the high radiation fields in the process liquids as generated by fission products therein which damage the extraction solvents over extended periods of contact. Also, with small inventories criticality concerns with respect to the fissionable material in the process liquids are minimized and emulsification problems are significantly controlled by the large separating force applied to the process liquids in the centrifugal contactor.
In nuclear reactor fuel reprocessing operations, the centrifugal contactors can be utilized to separate uranium and plutonium values from fission products and other actinides undesirable for use nuclear reactor fuels. Typically, in a nuclear fuel reprocessing operation using centrifugal contactors, a cascade of several centrifugal contactors are employed with each centrifugal contactor comprising a housing containing a vertically oriented rotor. The process liquids comprise a heavy solvent or organic phase and a lighter aqueous phase which are introduced into the centrifugal contactors through separate conduits or inlets which are in registry with a mixing zone before the liquid mixture enters the rotor where centrifugal force is utilized to separate the heavy phase from the lighter phase by forcing the heavy phase to flow outwardly away from the rotational axis of the rotor and displace the lighter phase closer to the rotational axis of the rotor. These process streams are then individually collected at the upper end of the rotor at a location adjacent to the outer periphery thereof for the heavier liquid phase and at a location adjacent to the rotational axis of the rotor for the lighter liquid phase. During this introduction of the process liquids into the mixing zone, the two phases are introduced tangentially into a mixing zone to enhance mixing and mass transfer between the liquid phases so that a good mixture of the liquid phases enter the rotor through an opening in the base thereof due to pressure gradients developed by the rotation of the rotor. In nuclear reactor fuel reprocessing systems, the centrifugal contactors, which may employ the improvement provided by the present invention, are disposed in a multistage or cascade arrangement with each contactor having a diameter of 5.5 centimeters. A detailed description of the use of centrifugal contactors in a multistage nuclear reactor fuel reprocessing operation is set forth in a report entitled Developments in Centrifugal Contactor Technology, R. T. Jubin et al, ORNL/TM-10768, U.S. Government Printing Office, September 1988. This report is incorporated herein by reference.
While the present invention is particularly useful in centrifugal contactors such as those envisioned for the use of reprocessing nuclear fuels, it is to be understood that the present invention may be utilized in centrifugal contactors employed for the separation of other liquids such as those generally mentioned above.
It has been found that while the use of centrifugal contactors in a multistage system are desirable for continuous countercurrent liquid extraction processes, a problem can occur during the flow of the process liquids between the various centrifugal contactors in that at relatively high flow rates such as when the one of the centrifugal contactors in the system ceases to operate. When this condition is present the liquids tend to flood the process liquid mixing zone of one or more of the centrifugal contactors coupled thereto and the process liquids seep into the discharge collection plenums of the contactor and contaminate the discharge, single-phase, streams of liquids with a two phase solution so as to significantly detract from the operation of the centrifugal contactors in the cascade. Another problem may also occur when relatively low flow rates of process liquids are entering the mixing zone. In such instances the level of the process liquids in the mixing zone falls below a level at which proper mixing and sufficient mass transfer can be achieved in the liquids before they are discharged from the mixing zone into the separating zone within the rotor. Still another problem with low liquid levels in the mixing zone is that vortices form in the process liquids in the mixing zones and introduce cavitation in the process liquids which significantly reduces the efficiency of the mass transfer between the liquid phases and also introduces air into the liquid process stream which has a deleterious effect upon the liquid distribution and countercurrent flow of the process liquids. In view of these problems, it was found that in order to satisfactorily operate a multistage centrifugal contactor system, a relatively restricted and narrow range of flow rates for the process liquids was required so as to minimize some of the problems associated with excessive or relatively low flow rates of process liquids. Even with this restriction on the flow rates of process liquids the problems occurring when one or more centrifugal contactors in the cascade fail can not be readily compensated for or overcome.